The present invention relates to an objective lens drive device of an optical pickup constituting optical disk apparatus that can radiate an optical spot on a recording medium on a disk to optically read information therefrom.
(Related Art 1)
An optical pickup constituting optical disk apparatus is generally composed of an object lens drive device equipped with an objective lens and an optical system for transmitting/receiving light to/from the objective lens, the object lens drive device is arranged on a stand for mounting an optical system block. The object lens drive device is composed of a movable part equipped with an objective lens, a focus coil and tracking coils, and a fixed part equipped with a magnetic circuit. The movable part is supported by the fixed part via four wires partially enclosed/held with an elastic damper material such as a viscoelastic material.
(Related Art 2)
In a first related art, the movable part is not inclined when it is moved in the focusing direction, because the position of center of gravity of the movable section coincides with the center of drive device in the focusing direction. Next, when the movable part is moved in the tracking direction, the position of center of gravity in the movable part moves the same amount as the tracking moving amount. However, the position of center of drive device in the focusing direction can be assumed as the center of the magnetic circuit in the fixed part so that moving in the focusing direction while moving in the tracking direction causes dislocation between the position of center of drive and the position of center of gravity in the movable part in the focusing direction thereby generating an angular moment around the center of gravity. This generates an inclination of the movable part in the tracking direction.
As a means for solving these problems, an optical pickup is known wherein a movable part including an objective lens is simultaneously driven in the focusing direction and tracking direction, and, in case the center of drive of focusing is dislocated from the center of gravity of the movable part, the rotation moment generated on the movable part is made symmetrical about the center of gravity (for example, Japanese Patent Publication No. Hei. 8-50727).
As shown in FIG. 19, the optical pick up is equipped with magnet holding pieces 208a, 208b, and facing yokes 209a through 209d provided inside the magnet holding pieces and split in the tracking direction so that these pieces face each other, on a base 202 as a fixed base. Via this configuration, as shown in FIGS. 20A and 20B, the magnetic flux density distribution of the magnetic circuit for the focusing drive coil 210 is shaped in twin peaks. This maintains the balance of the rotation moment even in case the movable part including the objective lens 203 is dislocated in the tracking direction and focusing direction, thus preventing inclination of the optical axis of the objective lens 203.
In FIGS. 19 and 20, 201a and 201b represent magnets, 204 represents a lens holder, 205 represents a tracking drive coil, 206a trough 206d represent support members, 207 represents a fixed member, X represents a tracking direction, Y represents a direction perpendicular to a focusing direction and the tracking direction, and Z represents the focusing direction.
(Related Art 3)
In the objective lens drive device according to the first related art, a single magnetic circuit is used and tracking coils are glued on the focus coil and arranged in a magnetic gap in order to downsize and making more low-profile the drive. However, the objective lens drive device according to the related art had a problem that the center of a drive force via a focus coil in the focusing direction was dislocated from the center of a drive force via tracking coils in the tracking direction and making the center of gravity of the movable part coincide with the center of one driving force dislocates the other, and a moment is generated according to the dislocation amount, thus causing unwanted resonance.
As a means for solving these problems, an optical pickup is known wherein a focus coil is arranged to pinch a tracking coil or a tracking coil is arranged to pinch a focus coil in order to cause the center of each drive force to coincide with each other (for example, Japanese Patent Publication No. Hei. 6-124467).
As shown in FIG. 21, the focus coil 310 is fixed to the top and bottom sections of a bobbin 312 via adhesion and the a tracking coil 311 is fixed to the center of the bobbin 312 via adhesion in a position it is pinched by the focus coil 310. As shown in FIG. 22, the focus coil 310 is fixed to the center of the bobbin 312 via adhesion and the tracking coil 311 is fixed to the focus coil 310 via adhesion in a position it pinches the focus coil 310 from top and bottom, the respective coils placed in a yoke 309 that fixes a permanent magnet 309 via adhesion. The optical pickup allows the center of a drive force in the focusing direction (arrow A) and in the tracking direction (arrow B) to coincide with the center of gravity of the movable part by feeding a current across both coils 310, 311.
(Related Art 4)
In the objective lens drive according to the first related art, it is required to raise a high-order resonance frequency in order to support high-speed optical disk apparatus. As a means for solving this problem, an objective lens drive device is known wherein the objective lens attachment section of the lens holder is formed to gradually get thinner toward the free end so that a high-order resonance frequency may be raised (for example, Japanese Patent Publication No. Hei. 8-194962).
As shown in FIG. 23, the lens holder 411 is formed with its bottom face tapered so that the objective lens attaching section 411c may become thinner toward the free end (on the left in the figure) of the lens holder 411 and that the objective lens attaching section 411c becomes thicker toward the fixed part (on the right in the figure). The objective lens attaching section 411c has a thickness b at the front edge and a thickness c at the rear edge.
In the objective lens drive device according to the second related art, a problem arises that a magnetic circuit must have a complicated shape in order to provide twin-peak magnetic flux distribution of the magnetic circuit for the focusing drive coil 21. Even in such a configuration, it is difficult to sufficiently downsize the moment. This leads to a problem that moving the movable part in the tracking direction causes the inclination of the movable part and generates coma aberration, thus worsening the read signal jitter.
Further, in the objective lens drive device according to the third related art, a problem arises that a side facing the side arranged in the magnetic gap of the focus coil 310 is arranged outside the magnetic gap thus influenced by a leak magnetic flux from the magnetic circuit and generating a drive force and a moment in the opposite direction, resulting in that it is difficult to make the center of gravity of the magnetic circuit coincide with the center of the actual driving force.
In the objective lens drive device according to the fourth related art, a problem arises that, since the fixed part of the objective lens attaching section 411c of the lens holder 411 is formed to gradually become thicker, a mirror (right angle prism) arranged beneath the objective lens 411b must be lowered as the fixed part of the objective lens attaching section 411c is made thicker, thus preventing a low-profile optical pickup.
The invention aims at solving the above problems in the related art.
Means for solving the problems will be explained using FIG. 1 corresponding to an example of the invention. According to the invention, a coil unit on which a focus coil 3f and tracking coils 3t are formed is arranged in a magnetic gap 5g of a magnetic circuit containing at least one magnet 5 polarized on two poles in the focusing direction F and the magnet 5 and the tracking coils 3t are arranged so that the boundary 5b between the north pole and the south pole of the magnet 5 polarized on two poles in the focusing direction F falls within the width formed by the horizontal sides B (D) of the tracking coil 3t in the focusing direction F, the horizontal sides B (D) are perpendicular to the focusing direction, when the coil unit 3 is moved in the focusing direction F.
In such a configuration, the magnet 5 is formed on two poles polarized in the focusing direction F so that the magnetic circuit has a simple shape. On the tracking coil 3t is generated a moment having an approximately equal quantity to that of a moment caused by a thrust Df of a focus coil 3f in the focusing direction F and in the opposite direction to that of the latter moment. This sufficiently reduces the inclination of an objective lens 2 in the tracking direction T.
Means for solving the problems will be explained using FIG. 8 corresponding to an example of the invention. According to the invention, a focus coil 33f and tracking coils 33t are attached to a coil seating 31 with the center in the coil thickness direction matched and the focus coil 33f is arranged in a magnetic gap 5g of a magnetic circuit containing one magnet 5 polarized on two poles in the focusing direction F.
In such a configuration, the center of a drive force generated in the focusing direction F coincides with the center of a drive force generated in the tracking direction and the center of gravity of the movable part. The focus coil 33f generates only drive forces in the same direction.
Further, means for solving the problems will be explained using FIG. 16 corresponding to an example of the invention. According to the invention, a lens holder 1 is formed of a resin containing a light metal or a carbon fiber with high flexural elasticity modulus and a coil unit 3 is attached in two notches 1a of a lens holder formed in the tracking direction T.
In such a configuration, the elasticity ratio of the lens holder 1 becomes higher thus elevating the high-order resonance frequency. Thus, the mirror arranged beneath the objective lens 2 need not be lowered.